Atomic Spectra

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Atomic Spectra

• Rutherford Model - Atom is mostly empty space
  with all the positive charge concentrated in a
  tiny massive central core. He also suggested a
  planetary model where electrons orbit the
  nucleus. Although a major step forward, this
  model was flawed.
• Heated solids, liquids and dense gases emit light
  with a continuous spectrum of wavelengths. Less
  dense gases emit a discrete spectrum (emitted
  light is due to individual atoms, not
  interactions between atoms).

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• Emission Spectrum - when energy is transferred
  to atoms, the atoms absorb this energy and then
  emit it in the form of light. The spectrum of a
  gas is a series of lines of different colors,
  each line corresponding to a specific wavelength
  of light emitted from the atoms of the gas.

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• Absorption Spectrum - a gas that is cool will
  absorb certain wavelengths of light that is shone
  on it. A spectrum will show dark lines where
  wavelengths have been absorbed.

• Cool gaseous elements absorb the same wavelengths
  that they emit when excited. These spectra serve
  as a key to the structure of the atom.

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Bohr Model

• Visible spectrum of hydrogen consists of four
  lines - red, green, blue, and violet. Rutherford
  model could not explain this, and also predicted
  an unstable atom.
• In 1911, Niels Bohr attempted to unite
  Rutherford's nuclear model with Einstein's
  quantum theory.
• Rutherford focused on the nucleus, Bohr focused
  on the electrons.
• He suggested that the energy of an electron (and
  its radius) is quantized, and also that the laws
  of electromagnetism do not hold inside the atom!

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• The electron can exist in different energy
  levels. Smallest energy level is the ground
  state. If an electron absorbs energy, it makes a
  transition to an excited state however, it
  usually remains in this state for only a fraction
  of a second.
• Light is only emitted when an electrons drops to
  a lower energy state.
• The change in energy of an electron when a photon
  is absorbed or emitted is equal to the energy of
  the photon

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• Bohr derived an equation for the energy of an
  electron in an atom

• where n is called the principal quantum number
  (the radius increases with n2, the energy depends
  on 1/n2)
• When changing energy levels, electrons can jump
  directly or in steps for example, going from n3
  to n1 state, electron can go from 3 to 1, or
  from 3 to 2 and then from 2 to 1.

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• As a result, three different photons could be
  emitted in this example (each of whose energy is
  equal to or less than the energy that the
  electron originally absorbed)
• Fluorescence objects emit visible light after
  absorbing ultraviolet light (fluorescent lights)
• Phosphorescence electrons can be raised to
  metastable states these states last much longer
  than higher energy levels in typical atoms
  (luminous watch dials)

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Quantum Model of the Atom

• Bohr model calculated the emission spectrum and
  ionization energy of hydrogen atom, determined
  energy levels of the elements, and explained some
  of the chemical properties of the elements
  however
• His postulates could not be explained on the
  basis of known physics and only worked for
  hydrogen
• de Broglie suggested that each electron in the
  atom is actually a standing wave. This implies
  that wave-particle duality is at the root of
  atomic structure

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• Schrodinger and Heisenberg used de Broglie's wave
  model to begin a quantum theory of the atom.
• Radius of the orbit of the electron not the same
  as the radius of planet around the sun, but is
  actually much harder to visualize.
• Quantum model of the atom only predicts the
  probability that an electron is in a specific
  location. The region in which there is a high
  probability of finding the electron is called the
  electron cloud.
• Philosophic Implications - probability (Quantum)
  vs determinism (Newtonian)

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• Heisenberg Uncertainty Principle - position and
  momentum of a particle cannot both be precisely
  known at the same time
• Quantum model predicts the same energy levels as
  Bohr for hydrogen. Bohr model only had one
  quantum number (principal) quantum model uses 4
  quantum numbers (principal, orbital, magnetic,
  spin)
• Quantum mechanics uses this model to predict many
  details about the structure of the atom -- it
  takes powerful computers to calculate accurate
  details for many atoms